1. Field of the Invention
The present invention relates to an electromagnetic type contactless ignition apparatus for an internal combustion engine of a vehicle or the like.
2. Description of the Prior Art
In a known type of electromagnetic type contactless ignition apparatus, the "on" period for the primary winding of an ignition coil is directly determined in accordance with the output signal of an electromagnetic pickup having a frequency corresponding to the rotational speed of an internal combustion engine. A disadvantage of this conventional apparatus is that at low engine speeds the "on" period of the primary winding becomes excessively long and the heat generation of the ignition coil or the wasted amount of power is increased, while at high engine speeds the "on" period of the primary winding becomes excessively short thus making it impossible to produce satisfactory ignition sparks.
In known apparatus of this type proposed to overcome the foregoing difficulty, as for example disclosed in the specification of U.S. Pat. No. 3,605,713, a resistor is connected in series with the primary winding of an ignition coil and the current flowing in the primary winding is detected in accordance with the voltage developed across the terminals of the resistor, whereby when the detected current is higher than a predetermined value, the bias level of the electromagnetic pickup is changed to control the length of time during which the current flows in the primary winding of the ignition coil.
A disadvantage of this type of apparatus is that not only the construction of the apparatus tends to become complicated due to the fact that the primary winding current is detected by means of the resistor, but also due to the fact that the current detecting resistor is always inserted in the energizing circuit for the primary winding, the resistor generates heat exerting detrimental effect on the various circuits and moreover the power consumption is increased and the amount of secondary voltage is limited.